This name was instituted by Heer[1049] for pieces of pinnate fronds from Lower Cretaceous rocks of Greenland. The resemblance of the long pinnules to the fertile segments of _Laccopteris_ is so close that generic identity might well be assumed, but it has recently been shown by Nathorst[1050] that the soral characters justify Heer’s use of a distinctive name for the Arctic fern. The circular sori arranged in two rows (fig. 294, A, B) are superficially identical with those of _Laccopteris_, but consist of concrescent sporangia forming a circular synangium (fig. 294, C, D) like those of _Kaulfussia_ and _Ptychocarpus_. The lighter areas in fig. 294, D, represent the sporangia: fig. C shows the radial disposition of the numerous sporangial compartments round a central receptacle. From a stout midrib lateral veins arise at right angles, but their distal terminations are not preserved. It is probable, as Nathorst suggests, that Bayer’s[1051] species _Drynaria fascia_ from the Lower Cretaceous rocks of Bohemia should be referred to Heer’s genus. In the absence of well-preserved sori it would be exceedingly difficult, or even impossible, to distinguish between pinnules of _Laccopteris_ and _Nathorstia_. A Tertiary species, _Marattia Hookeri_ (fig. 261, C, p. 350), described by Gardner and Ettingshausen[1052] from the Eocene beds of the Isle of Wight is referred by them to the Marattiaceae because of a resemblance of the sterile pinnae to those of _M. Kaulfussii_; but this is insufficient evidence of relationship. CHAPTER XXIII. Psaronieae. This family name, first suggested by Unger, may be conveniently adopted for the numerous species of petrified tree-fern stems characteristic of the Lower Permian and Upper Carboniferous strata. In his monograph _Über die Staarsteine_ published in 1854, Stenzel[1053] referred to the Psaronieae as a special subdivision of the Filices most nearly allied to the Polypodiaceae. There is now a consensus of opinion in favour of including _Psaronius_ in the Marattiales, or at least of regarding the genus as more closely allied to the Marattiaceae than to any other family. While admitting that the balance of evidence is in favour of this view, it is probably wiser to retain the distinctive term Psaronieae on the ground that species of _Psaronius_ differ in several respects from any recent ferns, and because of our comparative ignorance in regard to the nature of the fructification. _Psaronius._